1. Field of the Invention
The present invention generally relates to a technology for demodulating a received signal, and particularly relates to a receiver for demodulating a signal which is transmitted by the Orthogonal Frequency Division Multiplexing (OFDM) method (or an OFDM signal or symbol).
2. Description of the Related Art
In wideband wireless communications, or the next-generation mobile communications currently being studied within the technical field, a system which provides for a multi-path propagation environment needs to be built. The multi-carrier modulation method suppresses an effect of selective frequency fading, which becomes particularly problematic in the multi-path propagation environment, by using a plurality of carriers (or sub-carriers) in a predetermined transmission band so as to transmit signals in parallel. The OFDM method in particular adds a Guard Interval (GI) between symbols which are effective. Hereby, for a multi-path delayed wave within the GI duration, effective suppression, of inter-symbol interference, modulation without using equalization, and effective handling of the multi-path fading are enabled. On the other hand, the delay spread differs greatly from one communications environment to another. For example, even if the delay amount were about 0.2 to 2.0 μs in an urban area, it may reach 10 to 20 μs in a hilly terrain or a basin. Therefore, from such a point of view, a guard interval having a length which is sufficiently long to subsume all delayed waves which arrive following the preceding wave should be set up.
However, as the guard interval is a redundant symbol, there is a need to maintain the ratio between the guard interval and the effective symbol duration at a predetermined level or above by increasing the whole OFDM symbol duration so as not to decrease the transmission efficiency while setting the long guard interval. However, as the OFDM symbol length is increased, the fading within one symbol duration will no longer be constant so that it will be less immune to fading. Furthermore, with the increase in the OFDM symbol duration (T-S), the sub-carrier spacing (Δf=1/T-S) will become smaller so that it will be less immune to Doppler shift and also the peak-to-average power ratio will increase (a performance degradation due to non-linear distortion will take place). Therefore, it is common to set up the guard interval having an appropriate length so as to separately perform some compensation for the delayed wave which arrives at a delay exceeding the guard interval duration.
In the Non-Patent Document 1, at the time of the Fast Fourier Transform (FFT) in the demodulation processing, filtering is performed in the time domain on the portion causing interference and a Maximum-Likelihood Sequence Estimation (MLSE) is performed, in order to suppress the ISI (Inter-Symbol Interference) which affects the whole band used (refer to Patent Document 1 for an example of other related-art methods).
Non-Patent Document 1
Suyama, “OFDM reception method in multi-path environment having delay profile exceeding guard interval”, Technical Report of the Institute of Electronics, Information and Communication Engineers RCS 2001-175, November 2001
Patent Document 1
JP11-298434A
However, according to a technology such as discussed in Non-Patent Document 1, a Viterbi equalizer having M2 states (where M is a modulation index) is needed per sub-carrier. Therefore, the method is disadvantageous from such points of view as circuit size, computational complexity, and power consumption of the receiver. It is particularly disadvantageous for use in a mobile communications device which needs to be kept small.
Incidentally, a study is currently underway on a communications system which seeks to improve the signal transmission efficiency using an adaptive modulation. As the provision of a receiver circuit which is adapted to the largest modulation index M is needed when the related-art technology is used in the communications system, the related-art method is also disadvantageous from the point of view that embedding in the adaptive modulation system is difficult.
Furthermore, the related-art method is also disadvantageous as a hard decision of a demodulated signal is made by performing the MLSE so that, likelihood information for the soft-decision information, is lost and the error correction technology cannot be utilized 100%.